JP2014038173A - Optical modulator - Google Patents

Abstract

To reduce the length of an optical modulator in the longitudinal direction.
A polarization demultiplexer / multiplexer having two inputs and two outputs, and a light having a ratio of TE polarization to TM polarization of 1: 1, the polarization demultiplexer / multiplexer having two inputs and two outputs. Connected to each of the two output ports of the two-input two-output polarization demultiplexer / multiplexer, and outputs either one of TE polarized light or TM polarized light. Two light modulation circuits that strongly modulate, a λ / 4 wavelength plate connected to the output of each light modulation circuit, and a mirror connected to the output of the λ / 4 wavelength plate, the two-input two-output The polarization multiplexed light in which the modulated TE polarized light and the modulated TM polarized light are polarization multiplexed is output from the other input port of the polarization demultiplexer / multiplexer.
[Selection] Figure 2

Description

  The present invention relates to downsizing of an optical modulator.

  In recent years, with an increase in communication traffic, high-capacity transmission for transmitting more data per optical fiber is required in a trunk optical transmission network. As means for realizing this, a multi-level modulation technique and a digital coherent reception technique for realizing improvement of frequency utilization efficiency and long-distance transmission have attracted attention. In the multi-level modulation method, it is essential to realize a high-performance optical modulator in consideration of the phase of light.

An optical modulator is a basic device for optical communication that converts an electrical signal into a light intensity signal. Generally, high speed, low loss, low power consumption, small size, and high reliability are required. Methods for realizing the optical modulator are classified into a direct modulation method and an external modulation method. In high-speed and backbone networks, the external modulation method is the mainstream in terms of high speed and long-distance transmission. As an optical modulator to which an external modulation method is applied, a dielectric material such as LiNbO ₃ (lithium niobate, hereinafter referred to as LN), a semiconductor material or an organic material utilizing an electro-optic effect (hereinafter referred to as EO), an electroabsorption effect The semiconductor material etc. which used is used.

  On the other hand, in a multi-level modulation type optical modulator, development of a PDM-QPSK (Polarization Division Multiplexed Quadrature Phase Shift Keying) type modulator that uses polarization of light is proceeding (for example, see Non-Patent Document 1). . Such a modulator needs to be provided with a passive optical circuit that demultiplexes and combines polarization multiplexed optical signals.

  FIG. 1 shows a configuration of a conventional PDM-QPSK modulator. FIG. 1A is a diagram for explaining the function of the modulator, and FIG. 1B is a diagram showing a configuration of the quartz-LN modulator. When TM polarized CW light is incident from the laser diode LD, it is demultiplexed by the demultiplexer and input to the two modulators QPSK1 and QPSK2. The optical signal modulated by the modulator QPSK1 is converted into a TE polarized wave by the polarization rotator HWP and input to the polarization beam combiner PBC. The optical signal modulated by the modulator QPSK2 is input to the polarization beam combiner PBC as TM polarization, and the optical signal obtained by polarization multiplexing the TE polarization and the TM polarization is output from the polarization beam combiner PBC. The

  The PDM-QPSK modulator is composed of an LN substrate LN constituting an LN waveguide of the modulators QPSK1 and QPSK2, and quartz PLC substrates PLC-L and PLC-R on which optical functional circuits such as multiplexers and demultiplexers are integrated. The Electrodes are formed along the LN waveguides of the modulators QPSK1 and QPSK2, and the optical signal is modulated by applying an electric field to the optical waveguide through which the optical signal is transmitted.

H. Yamazaki et al., "Integrated 100-Gb / s PDM-QPSK modulator using a hybrid assembly technique with silica-based PLCs and LiNbO3 phase modulators," Proc. ECOC 2008, Mo3.C.1 A. Sano et al., "240-Gb / s polarization-multiplexed 64-QAM Integrated modulation and blind detection using PLC-LN hybrid integrated modulator and digital coherent receiver," Proc. ECOC 2009, PD2-2

  In order to sufficiently bring out the EO effect of the LN modulator, it is necessary to provide several cm of electrodes along the optical waveguide, and the length of the substrate LN in the optical axis direction is inevitably about several cm. In addition, since an optical circuit for multiplexing / demultiplexing light is required on the input side and output side of the LN modulator, the length in the optical axis direction is further increased by the amount of the optical circuit. Conventionally, the lengths in the optical axis direction of the substrates PLC-L and PLC-R on which the optical circuit for multiplexing / demultiplexing light is mounted are around 20 mm. It is drawn briefly.

  Further, when the PDM-QPSK modulator is housed in a package, the optical fiber is buckled to a certain length between the connection portion between the optical fiber connected to the quartz PLC and the quartz PLC and the pipe portion of the package. An extra fiber length is required.

  Accordingly, the package that accommodates the PDM-QPSK modulator may have a length in the longitudinal direction of about 140 mm including the fiber boot portions at both ends. When mounted on a board in a communication apparatus, However, there is a problem that the restrictions on the implementation become large.

  An object of the present invention is to reduce the size of an optical modulator of a polarization multiplexing multilevel modulator having a complicated and huge configuration, such as a PDM-QPSK modulator, and in particular, to reduce the length in the longitudinal direction. Is to provide.

  In order to achieve such an object, an embodiment of the present invention includes a two-input two-output polarization demultiplexer / multiplexer, and light having a ratio of TE polarization to TM polarization of 1: 1. A light source that outputs to one input port of the polarization splitter / multiplexer with two inputs and two outputs, and a TE that is connected to each of the two output ports of the polarization splitter / multiplexer with two inputs and two outputs; Two optical modulation circuits that strongly modulate one of the polarized light and TM polarized light, a λ / 4 wavelength plate connected to the output of each optical modulation circuit, and the output of the λ / 4 wavelength plate The modulated TE polarized light and the modulated TM polarized light are polarized from the other input port of the 2-input 2-output polarization demultiplexer / multiplexer. The multiplexed polarization multiplexed light is output.

  Specifically, a first waveguide in which an input-side waveguide for inputting light from the light source, the 2-input 2-output polarization demultiplexer / multiplexer, and a multiplexing / demultiplexing portion of the modulation circuit are formed. A substrate and a second substrate optically connected to the first substrate and formed with a high-speed modulation portion of the modulation circuit, wherein the second substrate is connected to the first substrate. The λ / 4 wavelength plate and the mirror can be sequentially connected to the end face opposite to the end face.

  As described above, according to the present invention, it is possible to reduce the length in the longitudinal direction of an optical modulator of a polarization multiplexing multilevel modulator having a complicated and huge configuration such as a PDM-QPSK modulator. Can do.

It is a figure which shows the structure of the conventional PDM-QPSK modulator. It is a figure which shows the basic composition of the optical modulator concerning one Embodiment of this invention. It is a figure which shows the structure of the optical modulator concerning 1st Example of this invention. It is a figure which shows the structure of the optical modulator concerning the 2nd Example of this invention. It is a figure which shows the structure of the optical modulator concerning the 3rd Example of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 2 shows a basic configuration of an optical modulator according to an embodiment of the present invention. In the optical modulator 10, a light source 11 is connected to one input port of a 2-input 2-output (2 × 2) polarization splitter / multiplexer 12, and modulation circuits 13, 14 are connected to each of the two output ports. The λ / 4 wavelength plates 15 and 16 and the mirrors 17 and 18 are connected in cascade.

  The light source 11 outputs CW light (Continuous Wave light) in which the ratio of TE polarization to TM polarization is 1: 1, and the 2 × 2 polarization demultiplexer / multiplexer 12 is the polarization of CW light. , And TE polarized light is output to one output port and TM polarized light is output to the other output port. On the other hand, when TE polarized light is input to an output port from which TE polarized light is output, and TM polarized light is input to an output port from which TM polarized light is output, TE polarized light and TM are input to the input port to which CW light is input. Polarized multiplexed light combined with the polarized light is output. Conversely, if TM polarized light is input to the output port from which the TE polarized light is output and TE polarized light is input to the output port from which the TM polarized light is output, the input port is different from the input port from which the CW light is input. The polarization multiplexed light obtained by combining the TE polarized light and the TM polarized light is output. The terms “input” and “output” used in the 2 × 2 polarization demultiplexing / multiplexing unit are the operations of the 2 × 2 polarization demultiplexing / multiplexing unit at the time of polarization demultiplexing. This is referred to for the sake of convenience based on the input / output of light.

  As the light source 11, a semiconductor laser that generates TM-polarized CW light is combined with a polarization controller to tilt the polarization plane by 45 °, and the CW light with the tilted polarization plane is 2 × 2 polarization demultiplexer / multiplexer 12 can be used. Further, a semiconductor laser that generates TM polarized CW light is combined with a λ / 4 wavelength plate to convert it into circularly polarized light or elliptically polarized light, and the converted CW light is converted into 2 × 2 polarized wave demultiplexer / multiplexer 12. It can be set as the structure input into. Further, the CW light from the semiconductor laser may be guided by the polarization maintaining fiber, and the polarization axis of the polarization maintaining fiber may be inclined by 45 degrees at the connection to the optical modulator.

  As the 2 × 2 polarization splitter / multiplexer 12, for example, an optical waveguide type polarization beam splitter or a dielectric multilayer cube beam splitter disclosed in Non-Patent Document 1, a two-input two-output reversible A certain polarization beam splitter (hereinafter referred to as PBS (Polarization Beam Splitter)) can be used.

  The modulation circuits 13 and 14 connected to the output port of the 2 × 2 polarization demultiplexer / multiplexer 12 strongly modulate one of the TE polarized light and TM polarized light. For example, in a modulation circuit using an LN waveguide of a Z cut substrate, only TM polarization is modulated. As a specific configuration of the modulation circuit, an appropriate configuration is used according to the modulation format.

  The λ / 4 wavelength plates 15 and 16 connected to the output side of the modulation circuits 13 and 14 (the side opposite to the side where the 2 × 2 polarization demultiplexer / multiplexer is connected) are connected to the modulation circuits 13 and 14. Is rotated by 90 ° and reflected by the mirrors 17 and 18. More specifically, the TM polarized light is converted from circularly polarized light by the λ / 4 wavelength plate 15, reflected by the mirror 17, and again converted from circularly polarized light to TE polarized light by the λ / 4 wavelength plate 15. The TE polarized light is converted from circularly polarized light by the λ / 4 wavelength plate 16, reflected by the mirror 18, and converted again from circularly polarized light to TM polarized light by the λ / 4 wavelength plate 16. That is, by transmitting twice through the λ / 4 wavelength plate, the plane of polarization is rotated by 90 °, and the operation is equivalent to when transmitting through the λ / 2 wavelength plate.

  When such a configuration is used, the light whose polarization plane is orthogonal to the light output from the output port of the 2 × 2 polarization splitter / multiplexer 12 returns to the same output port. In addition, in the modulation circuits 13 and 14, light is modulated on either the forward path or the return path, and is not modulated on the other side. The light returning to the output port of the 2 × 2 polarization demultiplexing / multiplexer 12 is output as polarization multiplexed light to an input port different from the input port to which the CW light is input.

  Conventionally, a circuit for separating the polarization of input light on the input side of the modulator and a circuit for coupling the separated polarization on the output side of the modulator are required. According to the present embodiment, it can be configured by one 2 × 2 polarization demultiplexer / multiplexer. In addition, as shown in the examples described later, when the modulation circuit includes a passive optical demultiplexing unit and a multiplexing / demultiplexing unit, the optical multiplexing / demultiplexing can be performed with a single circuit by using a folded configuration as in this embodiment. I can also serve. Furthermore, since the connection part of the optical fiber used for input / output is integrated into one of the optical modulators, one of the extra fiber length portions conventionally provided on the input / output side of the modulator can be omitted. Accordingly, it is possible to shorten the length of the optical modulator in the longitudinal direction.

  In FIG. 2, in order to clarify the basic configuration, a λ / 4 wavelength plate and a mirror are provided in the subsequent stage of the two modulation circuits, respectively, but they are shared as described in the following embodiments. It is also possible to reduce the number of parts. In addition, although the output of the modulation circuit (side connected to the λ / 4 wavelength plate) is drawn by one line, the modulation circuit includes a passive optical multiplexing / demultiplexing unit as described in the following embodiments. In the case of multiple outputs. Also in this case, the λ / 4 wavelength plate and the mirror can be shared by all the waveguides.

  An example in which the configuration of the present embodiment is applied to a quartz PCL-LN modulator will be described. Such a multi-chip integrated device in which an optical functional member connected with a planar lightwave circuit (hereinafter referred to as PLC) is integrated is advantageous in terms of characteristics as follows.

  The optical characteristics of an optical functional member made of LN, a semiconductor material, an organic material, etc. are inferior from the viewpoint of low loss and connectivity with an optical fiber as compared with a glass material. On the other hand, if a PLC in which quartz glass is deposited on a Si substrate or the like is used, a passive optical circuit can be realized with low loss. Therefore, by utilizing the excellent optical characteristics of quartz PLC made of quartz glass-based material, by combining quartz PLC with an optical functional member made of dielectric material such as LN, semiconductor material, organic material, etc. You can take advantage of the advantages.

  In such an optical modulator, the optical input / output unit between the quartz PLC chip and the optical functional member chip is appropriately connected and integrated. A device in which two or more chips are integrated is handled as one device (hereinafter referred to as a multichip integrated device), and an optical fiber that performs optical input / output with the outside is connected to the multichip integrated device. As a typical example of an optical modulator using a multichip integrated device, a modulator combining a quartz PLC and an LN waveguide (hereinafter referred to as a quartz-LN modulator) is known (for example, non-patent). Reference 2).

  FIG. 3 shows the configuration of the optical modulator according to the first embodiment of the present invention. The optical modulator 110 is a multichip integrated device in which a quartz PLC substrate 111 and an LN substrate 112 are connected. In the figure, the LN substrate 112 is drawn extremely short for convenience. The quartz PLC substrate 111 is formed with a polarization beam splitter PBS and a multiplexing / demultiplexing unit (1 × 4 branching / coupling circuit) of the modulation circuits QPSK1 and QPSK2. The LN substrate 112 is formed with a high-speed modulation section of the modulation circuits QPSK1 and QPSK2, which includes an LN waveguide and a modulation electrode. The polarization beam splitter PBS of the present embodiment uses a 2-input × 2-output waveguide splitter (see, for example, Non-Patent Document 1), but a bulk splitter may be used.

  On the LN substrate 112, the LN waveguides of the modulation circuits QPSK1 and QPSK2 and the modulation electrodes along the LN waveguides are formed. One end face of the LN substrate 112 is polished, and the LN waveguide and the optical waveguide connected to the multiplexer / demultiplexer formed on the quartz PLC substrate 111 are optically connected. The other end surface is also polished, and the λ / 4 wavelength plate 121 and the mirror 122 are optically connected. The λ / 4 wavelength plate 121 is made of a polyimide thin plate, and the mirror 122 is a quartz glass plate deposited with a metal film. The LN substrate 112 and the λ / 4 wavelength plate 121 and the λ / 4 wavelength plate 121 and the mirror 122 are directly bonded using an optical adhesive such as an ultraviolet curable resin.

  TM polarized CW light is incident on the polarization controller 131 from a laser diode as a light source. The TM polarization whose electric field vibrates in the vertical direction with respect to the substrate plane of the quartz PLC substrate 111 is converted into a polarization inclined by 45 degrees by the polarization controller 131, and is input to one side of the polarization beam splitter PBS of the quartz PLC 111. Input to the waveguide. The polarization beam splitter PBS outputs TM-polarized CW light from one output-side waveguide, inputs it to the modulation circuit QPSK1, and outputs TE-polarized CW light from the other output-side waveguide, Input to the modulation circuit QPSK2.

  The TM-polarized CW light input to the modulation circuit QPSK1 is branched into four by the multiplexing / demultiplexing unit on the quartz PLC, and then modulated in the LN waveguide, and the modulated TM-polarized signal light is λ / 4. Input to the wave plate 121. The TM-polarized signal light is converted into circularly-polarized light in the λ / 4 wavelength plate 121, reflected by the mirror 122, and transmitted again through the λ / 4 wavelength plate 121, so that LN light is transmitted as TE-polarized signal light. Input to the waveguide. The TE-polarized signal light is not subjected to the modulation operation in the LN waveguide and is directly coupled by the multiplexing / demultiplexing unit on the quartz PLC substrate, and then input to one output-side waveguide of the polarization beam splitter PBS. Is done.

  The TE-polarized CW light input to the modulation circuit QPSK2 is branched into four by the multiplexing / demultiplexing unit on the quartz PLC, and then input to the λ / 4 wavelength plate 121 as it is without undergoing a modulation operation in the LN waveguide. Is done. The TE-polarized CW light is converted into circularly polarized light in the λ / 4 wavelength plate 121, reflected by the mirror 122, and transmitted again through the λ / 4 wavelength plate 121, so that the LN light is transmitted as TM-polarized CW light. Input to the waveguide. The TM-polarized CW light is modulated in the LN waveguide, and the modulated TM-polarized signal light is coupled by the multiplexing / demultiplexing unit on the quartz PLC substrate, and then the other output of the polarization beam splitter PBS. Input to the side waveguide.

  The TE-polarized signal light from the modulation circuit QPSK1 and the TM-polarized signal light from the modulation circuit QPSK2 become signal light that is polarization-multiplexed in the polarization beam splitter PBS, and input light from the polarization controller 131. Is output from an input side waveguide different from the input side waveguide. The optical input unit may be configured such that the polarization controller 131 is omitted, and the input fiber (polarization maintaining fiber) and the quartz PLC 111 are connected by tilting the polarization axis of the input fiber by 45 degrees.

  According to the first embodiment, one quartz PLC substrate can be omitted as compared with the conventional optical modulator. Further, an input / output optical fiber can be connected to one end face of the quartz PLC substrate and taken out from one face of the package. Accordingly, the extra length of one optical fiber can be omitted, and the length of the package in the longitudinal direction can be greatly shortened. For example, in one embodiment, a package of about 140 mm in the longitudinal direction including the fiber boot portion at both ends can be shortened to about 100 mm including the fiber boot portion at one end.

  When mounting the optical modulator 110 on a board in a communication apparatus, restrictions on mounting can be relaxed in connection with other devices. Conventionally, after mounting an optical modulator on a board, it has been necessary to route the optical fibers coming out from both ends of the package to connect to other elements on the board. The term “handling” refers to connecting the elements while bending the optical fiber at a certain radius R or more on the board. Therefore, when a conventional package is used, a space for routing more than a radius R in the longitudinal direction of the package is required on the board. According to the first embodiment, one of the spaces for such handling can be omitted. Therefore, it can also contribute to size reduction of the board in the communication device.

  As shown in the first embodiment, when an LN modulator is used as an optical functional member, a multichip integrated device having a modulation function equivalent to that of a conventional package size is realized while the package is shortened. It was confirmed that it was possible. Propagation loss due to the modulated signal light or CW light turning back through the LN waveguide is 1-2 dB, and the loss at the connection portion of the λ / 4 wavelength plate 121 is about 1 dB, which has no practical effect.

In this embodiment, even if an LT modulator made of a dielectric material other than LN, for example, LiTaO ₃ (hereinafter referred to as “LT”) is used as the optical functional member, the same shortening and modulation function can be realized. Can do. Furthermore, even if a GaN modulator made of GaN, which is a semiconductor material, and an InP modulator made of InP are used, the same effect can be obtained while utilizing their high EO efficiency, and an organic EO material is used. However, an equivalent effect can be obtained without reducing the advantage of the high-speed response.

  The optical functional member is not limited to the modulation function shown in the first embodiment, and for example, an EO switch waveguide made of an organic material or a semiconductor material, or a thermo-optic switch waveguide made of Si can be used. Even in these cases, it is possible to shorten the package in the longitudinal direction while maintaining the optical switch function.

  FIG. 4 shows the configuration of an optical module according to the second embodiment of the present invention. The difference from the first embodiment is that the connection method of the λ / 4 wavelength plate 221 and the mirror 222 is different. The same quartz PLC substrate 211 as in the first embodiment is connected to one end face of the LN substrate 212, and the quartz PLC substrate 213 is also connected to the other end face. On the quartz PLC substrate 213, an optical waveguide optically connected to the LN waveguide formed on the LN substrate 212 is formed. A groove is formed across the plurality of optical waveguides, and a λ / 4 wavelength plate 221 and a mirror 222 are inserted into the groove.

  In the first embodiment, the λ / 4 wavelength plate 121 is directly attached to the LN substrate 112. However, in the second embodiment, the λ / 4 wavelength plate 221 and the mirror 222 are attached to the quartz PLC substrate 213. , An equivalent configuration can be obtained.

  FIG. 5 shows the configuration of an optical module according to the third embodiment of the present invention. The difference from the first embodiment is the connection method between the λ / 4 wavelength plate 321 and the mirror 322. The same quartz PLC substrate 311 as that of the first embodiment is connected to one end face of the LN substrate 212, and a groove is formed so as to cross a plurality of LN waveguides formed on the LN substrate 312 near the other end face. And a λ / 4 wave plate 321 and a mirror 322 are inserted into the groove.

  In the first embodiment, the λ / 4 wavelength plate 121 is directly attached to the LN substrate 112. In the third embodiment, the λ / 4 wavelength plate 321 and the mirror 322 are attached to the groove of the LN substrate 312. Thus, an equivalent configuration can be obtained.

  Also in the second and third embodiments, the longitudinal direction can be shortened as in the first embodiment.

  In the first to third examples, the configuration of this embodiment has been described using a multichip integrated device called a quartz PCL-LN modulator. In addition to this, for example, an optical circuit corresponding to an optical circuit of quartz PLC in addition to the LN modulator may be mounted on one LN substrate, and the configuration close to monolithic may follow the configuration of this embodiment. It will be a thing. Alternatively, a discrete PBS may be prepared, and the PBS and LN modulator may be optically coupled by a spatial optical system.

LD Laser diode 13, 14, QPSK1, QPSK2 Modulation circuit HWP Polarization rotator PBC Polarization beam combiner LN, 112, 212, 312 LN substrate PLC-L, PLC-R, 111, 211, 311 Quartz PLC substrate PBS Polarization Beam splitter 10, 110, 210, 310 Optical modulator 11 Light source 12 Input 2 output (2 × 2) Polarization demultiplexer / multiplexer 15, 16, 121, 221, 321 λ / 4 wavelength plate 17, 18, 122 , 222, 322 Mirror 131, 231, 331 Polarization controller

Claims (3)

2-input / 2-output polarization splitter / multiplexer;
A light source that outputs light having a ratio of TE polarization to TM polarization of 1: 1 to one input port of the two-input two-output polarization demultiplexer / multiplexer;
Two optical modulation circuits connected to each of the two output ports of the two-input two-output polarization demultiplexer / multiplexer for strongly modulating one of TE polarized light and TM polarized light;
A λ / 4 wave plate connected to the output of each light modulation circuit;
A mirror connected to the output of the λ / 4 wave plate,
From the other input port of the 2-input 2-output polarization demultiplexer / multiplexer, polarization multiplexed light in which modulated TE polarized light and modulated TM polarized light are polarization multiplexed. An optical modulator that is output. An input-side waveguide for inputting light from the light source, the 2-input 2-output polarization demultiplexing / multiplexing unit, and a first substrate on which a multiplexing / demultiplexing unit of the modulation circuit is formed;
A second substrate optically connected to the first substrate and formed with a high-speed modulation section of the modulation circuit;
The λ / 4 wavelength plate and the mirror are sequentially connected to the end surface of the second substrate opposite to the end surface connected to the first substrate. Light modulator. The optical modulator according to claim 1, wherein the first substrate is made of a glass-based material, and the second substrate is made of LiNbO ₃ .

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